Seismometer system



Nov. 27, 1951 F. D. CASE 2,576,775

SEISMOMETER SYSTEM Filed July 31, 1948 5 sheets-Sheet 1 Nov. 27, 1951 F. D. CASE 2,576,775

\ SEISMOMETER SYSTEM Filed July 51, 1948 5 Sheets-Sheet 2 FBANN QSL JNVENTOR.

F. D. CASE SEISMOMETER SYSTEM Nov. 27, 1951 Filed July 3l, 194B 5 Sheets-Sheet 5 INVENTOR.

Nov. 27, 1951 F. D. CASE 2,576,775

SEISMOMETER SYSTEM Filed July 31, 1948 5 Sheets-Sheet 4 F'fimlcwa INVEN TOR.

Nov. 27, 1951 Filed July 5l, 1948 FIG.

F. D. CASE SEISMOMETER SYSTEM 5 Sheets-Sheet 5 FRANK C^5E INVENTOR,

Patented Nov. 27, 1951 SEISMOMETER SYSTEM Frank n. case, needing, Msgs., migrar-i mamond Instrument Company, Wakeeld, Mass.,

a corporation s claims.

This invention pertains to improvements in seismometers and comprises a system adapted to detect simultaneously a plurality of diverse earth movements. f seismometers have been designed and produced with a single column supporting a plurality of earth movement detecting components. In the present state of the science of earthquake detection, amplification and recording, it is not practicable to have more than one long-period seismometer suspended from the column because, for such detecting, the suspension means are so delicately balanced that the action of adjusting one of the components may throw ofibalance the others suspended Afrom the same supporting column and result in inaccurate amplification and recording. It is advantageous, however, to have such instruments a's compact as possible consistent with the results to be obtained, so as to avoid bulk and weight in transportation of the units. The advantages of my system over previous methods will be understood by those who are engaged in the manu` facture and use of such instruments.

My seismo detecting system includes two seismometers for` detecting horizontal earth movements and one for detecting vertical earth movements; these will be referred to hereinafter as the horizontal and vertical seismometers or components. Each seismometer has a horizontally disposed boom or pendulum. On each boom is a vane which is disposed between two conductive plates. The vane and plates constitute a transducer through which pulses indicating earth movements are transmitted through an electronic amplier to a recorder. Period adjustment, drift compensation, air and magnetic damping and other features arel included in the system.

In the drawings that accompany this application for a patent as a part thereof, Figure 1 is a side elevation of a horizontal seismometer; Figure 1a is a detail showing forked end of the suspension hinge adjusting screw; Figure 2 is a plan view of horizontal seismometer; Figure 3 is a side elevation of the vertical'seismometer; Figure 4 is a side view of the same component; Figure 5 is a side view of the cross-slide which supports the conductive plates of the transducer; Figure 6 is a view of the magnetic damping lay-out; Figure 7 is a detail of the suspension tration of the transduc control elements: Fig;-

hinge for horizontal components; Figure 8 illustrates the period adjusting elements; Figure 9 is a detail showing the calibrater mounted on the cross-slide; Figure 10 is a diagrammatic illus- Vas shown in the drawings.

ure 11 shows how all three components and their amplifiers may be positioned when in use. and also shows the recorder.

The amplifiers and the recorders are not shown in detail but are indicated in the drawings because the seismometers are adapted for use with any amplifying and recording means. The recorder may be positioned, at a distance from the seismo detecting station where the seismometers and amplifiers are set up.

Each seismometer has an individual supporting column. Each column is vertically disposed on an adjustably horizontal base or platform to which it is rmly attached. The horizontal seismometers are identical in construction and a description of one of them will apply to the other. The vertical seismometer will also be described. When'the three seismometers are in use as a system, the'horizontal components are positioned so that their pendulum booms, when at rest, are perpendicular to each other, as, for example, in N-S, E-W positions. The supporting column of each seismometer is positioned near one side of and on'the longitudinal axis of the platform, which is preferably oblong in shape, Each seismometer is enclosed in a heat reecting casing which is indicated by the letter K.

The platforms l are mounted on adjustable feet 2, each of which includes a base having a conical legextending upward in a bore in the platform. The top of each bore is enlarged and threaded to engage a micrometer screw 3, the lower end of which is concaved to engage the top of the cone which is rounded o at the top. The screw is turned by means of knob 3 to make adjustments of the platform level. locknut 3b holds the screw in adjusted position. Two of these platform adjusting devices are positioned opposite each other at the sides of the platform. A third support is positioned at the transducer end of the platform on the platform's 1ongitudinal axis.

Horizontal components A backplate 4 is aiiixed to column 5 on the side that faces the center of the platform. This side will be referred to hereinafter as the face of the column. The top of the column slopes downward and backward from its face. Secured to the top of the column is a period adjusting slide block 6, with a longitudinal bore in which is a slide 1. A micrometer screw 8 controls the movements of the slide. The front end of the slide extends beyond the face of the column at an upward angle from the horizontal and is bifurcated as shown in Figure 1. Arms 'IP- and 'Ib of the slide are bored. A checknut 9 makes contact with the slide through the slide block to hold the slide in adjusted position.

A swinging member I with a pin hole passing therethrough is positioned between the arms of the slide and is movably mounted on a pivot pin transversely disposed to the longitudinal axis of the column, thus positioning the swinging member so that it may move in a vertical arc. On the swinging member there is a shoulder I0 and an extension III", the latter having threaded holes therein. A clamp II, of the same size and shape as the extension has holes passing therethrough.

A fiat hinge I2, preferably of beryllium copper and .0015 inch thick, shaped substantially as shown in Figure 7, has holes passing therethrough. The screw holes in the extension and the holes in the clamp and hinge are positioned so that screws passing therethrough hold one end of the hinge securely between the extension and the clamp. The shoulder on the swinging member aids in positioning the clamp and end of the hinge accurately for the above described assembly. On the upper end of suspension rod I3 is a cap I4, held in position thereon by a pin passing through the rod and cap. An extension on the cap, a shoulder and clamp provide means for fastening the lower end of hinge I2 to the suspension rod in the same manner as described above.

The lower end of the suspension rod is threaded to receive a cross-barv I5 which is adjustably positioned on the rod by passing the rod through a bore in the center of the cross-bar and securing it in position by means of a knurled nut mounted on the rod. At each end of the cross-bar -is a bore. Arms I5 and iSd are secured to the crossbar by passing reduced threaded ends of the arms through said bores and securing them in position b v nuts which engage said threaded parts. The other end of each arm is slotted and bored. Fingers I5e and I Bf are positioned in the slots and fastened with screws.

As already stated, the backplate is afxed to the face of the column. The position of the backplate above the surface of the platform is determined by the space reouired for positioning other elements of the assembly on theplatform in relation to the boom. Aiiixed to the backplate is a bracket comprising arms I6, IGb and a cross support Ilic. Screws pass through the cross support and arms into the backplate to hold the bracket in xed position so that the longitudinal axis of the cross support is in the same vertical plane as that of the column. The cross support is recessed to receive the xed hinge clamp I'Id which is aixed thereto. An end of hinge I n is afxed to the hinge clamp, as by screws. The other end of the hinge has attached thereto a free hinge camp I9, to which is attached a mov@ able cross-arm 20, In the face of the movable cross-arm are a. small round indentation and an elongated indentation. both having the same depth of depression. The hinge is of at spring metal of the same material as hinge I2.

Reading from right to left in Figures 1 and 2: On the end of the horizontal boom 2l is a boom 'fork comprising a.crosssupport 22 with pointed arms 22 and 22h attached thereto. The arms of the boom fork are disposed transversely to the longitudinal axis of the column and are held in position by engagement with the cross-arm 20,

the point of one of the arms being positioned in u the round depression and the other in the elongated depression in the cross-arm. Extending from the boom fork for a predetermined distance, the surface 'of the boom is provided with a iine thread. A checknut counter-weight 23 and a solenoid armature are threaded for adjustment of their positions on the boom. Adjustment of the counterweight maintains the center of gravity of the boom in its axis of suspension.

Also mounted on the boom is the mass 25, which is preferably cylindrical and is positioned on the boom by passing the boom through a bore in the longitudinal axis of the mass. The mass is held in position on the boom by a sunk screw. At each end of the transverse axis of the mass is a stud 25" and 25", which are adapted to be engaged by the bores in fingers I5, I 5' of the suspension rod cross-bar. This engagement holds the boom in suspension from the hinge vI2 at the top of the column, the boom being pivoted horizontally on the free cross-arm 20 by engagement of the boom fork therewith, which engagement has already been referred to.

A damping vane 26 is attached to the boom in a horizontal transverse plane. This vane is positioned between magnets. The magnetic damping means includes four magnets, two of which, 21 and 21h, are positioned above and two, 23* and 28h, below the damping vane as shown in Figure 1. Each of .these pairs of magnets is provided with an equalizer 21 and 28. 'Ihe magnets are mounted in movable brackets which are slidabiy positioned on standards 29 and 29". Iocknuts and springs on the standards hold -the magnets in adiustabe relation to the damping vane.

The end of the boom beyond the damping vane is bifurcated to receive the boom vane 30, which is disposed vertically and is fastened to the boom, as by screws. The boom vane is perforated as indicated by the numeral 3l)l in Figure 1, and is positioned between conductive plates 3| and,

which are mounted on the cross-slide assembly shown in Figure 5. The conductive plates are adjustably mounted in parallel spaced relation to the vane and to each other to permit horizontal movement of the vane between them. The position of the vane with respect to each conductive plate determines the amount of radio freouency voltage between them. Changes in position cause changes in the voltage in direct ratio to the change oi' position. Thus, the variable movements of the vane resulting from earth movements set up electrical pulses in plates 3| and 32 which are transmitted to an amplifying system and from thence to a recording instrument.

The cross-slide 33, which supports plates 3l and 32, is mounted on a base 33* ailxed to the platform I and is held in slidable relation to the boom at right angles thereto by the head of screw 34 which is mounted in the base, the screw head being position in slot 33h in the slide. A vertical support 35, of nonconductive material, is aflixed to the cross-slide and supports conductive plate 3| A block 36 is adjustably mounted on the cross-slide, as by screws passing through elongated holes 36, 36h in the block and into the cross-slide. Secured to the block is the vertical support 31, of nonconductive material, and which supports conductive plate 32. Each conductive plate is provided with a conductive post or stud 3| and 32B. Secured to the base of the cross-slide is a face plate 38 and in the plate is a bore through which a micrometer screw 33 extends and engages a threaded bore in the crossasume Vertical component The column is positioned as shown in Figures 3 and 4. A b lock 50 is attached to the rear side of the column. The block is provided with vertical faces and also with horizontal faces. Attached to the vertical faces of the block are the ends of two vetrical hinges 5| and SIB; and attached to the horizontal faces are the horizontal hinges 52 and 52B, in parallel relation to each other and equidistant from the longitudinal center of the boom frame. The hinges are of thin metal as referred to in the description of the hinges used on the horizontal-components.

The boom frame includes two parallel bars 2 la and 2lb. which are tied together by cross-bars. Amxed to the boom frame is a vertical plate 53, to which'is attached a block 54 with vertical and horizontal faces, these faces being positioned with relation to the faces on block 50 so vthat the vertical faces on block 5t receive and hold in position the vertical hinges and the horizontal faces on block 54 do the same in regard to the horizontal hinges. 1

A locking device 55, mounted on block 58, is positioned in relation to the hinges so that, by turning screws 56 and 51 against springs 55 and 51B, the hinges are held in immovable position when the instrument is not in use. Y

Attached at the rear end of the boom frame and extending upward therefrom is a vertical post 58 which is threaded to receive a counter-weight 59 adjustably mounted thereon. The purpose of this counter-weight is to bring the center of gravity of the boom assembly into the horizontal plane of the hinges, this being essential in order to prevent horizontal motion of the earth from setting the boom assembly in motion.

Adjustably mounted von the boom frame and slidable thereon is a carriage 6U comprising two vertical plates 6i and 62 attached to each other Aby cross-bars 63 and S6, which also support a block 65 to which is attached a hook 66.

Also attached to the carriage is a mass 25, vwhich is held suspended below the boom frame. By moving the carriage along the boom frame, the mass may be positioned as desired to change the free oscillation period of the boom assembly and its supporting spring.

Extending upward from the boom frame and in vertical position is a plate 61, to which is attached a rod 68. A standard 69`supports a solenoid armature BB in operatively related position with relation to the free end of rod 68. Also mounted on the plate is a horizontal arm 16. which supports the damping vane 26 in vertical position above the boom frame. The boom vane 30 is attached to the free end of the arm and is disposed horizontally instead of vertically. Otherwise this vane is identical with the vane already described.

The damping vane is. positioned between strong magnets as described in relation to the horizontal components, the magnets being turned at a 90 degrees angle with the positions already described,

so that the damping vane is positioned between' in parallel adjustable relation thereto and to each other. These positions of the plates are obtained by turning the cross-slide assembly 90 degrees from its position shown in Figures 1 and 2. The correlated actions of the boom vane and the plates are the same as hereinbefore described.

except that the vane moves in the vertical instead of in the horizontal plane.

Mounted on the platform below the boom frame hinges is a windlass 1I. An electric motor (not shown) is adapted to operate worm gear 13 through connection 13. The motor is driven by the voltage system to actuate the windlass through the worm gear to automatically adjust the tension of the fine wire 1t. This wire is wound around the windlass, passes over wheels 'i5 and 'I6 on top of the column to engage a helical spring 11, which is also attached to hook to hold the boom frame in horizontal suspension.

The cut threads, both internal and external, on the adjusting elements of all of the components are very fine, thus providing very close adjustments throughout the system.

Platform supports Period adjustments of horieontal components By means of the micrometer screw at the top of the column, the position of hinge i2 in relation to hinge i6 can be adjusted accurately. By reference to line B-C in Figure 8, it will be noticed that C, which represents the pivot point of hinge l2, is slightly forward of the pivot point B of the lower hinge. This increases the sensitivity ofthe boom to lateral motion because, the pivots being off-set vertically, the tendency of the boom to swing is increased. By means of the micrometer screw, to which the upper hinge is attached, the angular diiferential between the hinges may be adjusted, without affecting the horizontal position of the boom, for the purpose of changing--I the operating period of the instrument.

Heretofore, two common practices have been employed in the construction of horizontal pendulums for seismometers. In one case, provision is made for tilting the column represented by C-B in Figure 8, which changes angle 2 and thereby changes the period. The boom is then levelled by adjusting the length of C A, consisting of fine wire. In the other case, provision is madefor tilting the base or'platform to which the column is rigidly aixed, which also changes the period. The pivoting means are constructed ruggedly enough to withstand the vertical forces and the angle of the boom with the horizontal is neglected.

I1; is important in the construction of a seismometer pendulum to have the boom, represented by A-B in Figure 8, level at all times and to have the boom suspended through an axis about which the longitudinal weight moments are equal. When the boom is suspended in the above manner the force acting along the boom toward the hinge or pivot point B lies entirely within the horizontal plane. If the forces were not in the horizontal plane at all times, a exible hinge would be distorted by the vertical forces and, in the case of a pivot and jewel construction, for example, the pivot would tend to ride either up or down in the jewel and not directly in the center of the jewel where the optimum operating conditions are to be realized.

As shown in Figure 8, when the boom A-B, pivoted about the fixed point B, is suspended by a. member A--C having a fixed length but being free to rotate about the pivot A. as provided in my construction, point C may be supported at any point along the arc S--S' and the position of boom A-B will not be changed, the point C moving through such a small distance in making the necessary period changes that point C may be moved in a straight line T T', tangent to the arc S-S' at the center of the operating range.

Calibration of recordings The pointer 4|, mounted on micrometer screw 39 on the cross-slide, passes over a graduated segment 42 (see Figure 9) as the screw is turned,

thus providing a calibrating means for the electronic system A and recorder R. By this calibrating means, the conductive plates on the seismometers may be moved a known amount with respect to the boom vane. The linear displacement of the corresponding recording pen caused by turning the calibrating screw 39 may then be measured, and, when this measurement is divided by the known displacement of the plates, the result.is the static magnification ratio of the system.

Drift compensation The drift compensation devices overcome the very slight drift of the suspension springs that may occur during temperature changes at the higher manifestations. Also, the self-returning system indicated in Figure l0, While it may be included in the assembly of each of the seismometers in the system, is particularly important as part of the vertical component because it automatically compensates for the drift which occurs in all spring suspensions due to temperature changes and fatigue of the spring metals which heretofore have made. displacement type vertical long-period sesmometers of high magnification impracticable. Referring to Figure 10, the gear train is driven through a flexible shaft F by a reversible motor X which is wired to switches N and O. When the motor deviates from its center position, being that position at which the recording pen is in the center of the recording strip, an arm P attached to the motor shaft closes one of the switches, which drives the motor in a direction to either raise or lower the boom, thus bringing it back to the position at which the motor shaft no longer closes the switch. When the switch N is closed, field Y of the motor becomes energized and the motor will turn in one direction. It will turn in the opposite direction when field Q is energized by the closing of switch O.

Transducer adjustments The adjustments of the magnets of the damp-v ing assembly are easily made so that the magnets are kept in most eilicient relation to the pendulum.

Air dumping In order to prevent delayed movement of the pendulum vane between the transducer plates due to air bank between them. I have made apertures in the vane, as described hereinbefore, the apertures being of sumcient number and size and so positioned as to permit the air to pass through them freely and thus prevent delay of pendulum oscillations. i

The lulcrum hinge Amplifiers and recorder Each of the seismometers has its own amplifier or servochassis and when the seismometers are used in combination with each other the amplifiers transfer the pulses caused by earth movements to a single recorder through individual recording instruments or pens, by means of which the pulses from all of the detecting components are recorded on the same paper of the recorder. It is obvious, however, that the seismometers may be used singly, in which case each seismometer has its own recorder.

Positioning the recorder at a distance from the detecting station, as in the observers laboratory or workroom, enables him to obtain information at any time from the recorder roll without visiting the station.

Hurricane detection 'Ihe seismometers described in this specification are so delicately balanced that they are able to pick up earth tremors caused by hurricanes and record them in the same manner as earthquake `tremors are reported. The system is adapted to determine the intensity, distance from the detecting station and direction of movement of the hurricane so that it may be under constant observation from the moment that its existence is indicated on the recorder. In this case a plurality of horizontal components, as three, are used, set a mile or two apart, preferably at the points of aright-triangle and set in N-S, E-W and NW-SE detecting positions. with the recorder set up in the weather observatory or elsewhere away from the detecting instruments, if desired. In this way information of the hurricane and warnings can be given quickly and accurately.

Mines By adjusting the period to microseismos, a single seismometer with its amplifier and recorder, can be used to detect cave-ins and faults in mines.

The combined effect of the novel features disclosed results in more accurate detection of earth movements and more accurate transmission of pulses created thereby that has heretofore been provided as well as more speedy production of recorded tremors and more convenient reading of the records produced.

All of the adjusting knobs on the seismometers are of a special nonmetallic substance which minimizes the ow of heat from the operators hands 9 into the instruments while making adjustments during operation. The magnetic damping means and the solenoids are installed for obtaining decay curves at any time desired.

I claim: 1. A system of earth tremor detecting assemblies comprising, in combination, a plurality of Seismometers, `the seismometers being adapted to pick up earth tremors and transmit them through transducers and amplifying means to a recording assembly, each seismometer including a vertical column and a transducer aillxed to an adjustable horizontal platform, seismometer elements operatively suspended on each column for, respectively, detecting longitudinal, transverse and vertical earth disturbances, each assembly of said elements including a horizontally disposed pendulum, a vane on the pendulum adapted to be oscillated betweeen the poles of a transducer by movements of the pendulum, means for adjustably positioning the platform in the horizontal plane, means for adjustably suspending the seismometer elements on the column and on the pendulum, magneticI damping means mounted on the platform and adapted to be adjustably disposed in relation to the pendulum, the transducer poles being adjustably mounted in relation to the pendulum vane, air damping means including apertures in the vane, and electronic means adapted to maintain the elements of the assembly in accurate relation to each other.

2. In a device of the class described having a platform and column on which the elements of the device, including a pendulum, are mounted, means for suspending the pendulum for vertical oscillation thereof, said means including a bracket mounted on the column, a bracket mounted on the pendulum, said brackets being adapted to hold iu operative positions vertically and horizontally disposed hinges, an electrically controlled windlass mounted on the platform, pulleys mounted on the column, a wire attached to the windlass and passing over the pulleys, a

helical spring attached to the wire and to the pendulum, an adjustable counter-weight, a. mass and damping means mounted on the pendulum. a vane mounted on the pendulum in operative relation to transducer plates mounted on the platform, and means for connecting the transducer plates and platform with electronic amplifying means and with recording means.

3. In a device of the class described having a platform and column on 'which the elements of the device. including a pendulum, are mounted, means for pivoting the pendulum in horizontally suspended position for horizontal oscillation, said means including a backplate mounted on the column, a bracket, including a cross-support and arms, mounted on the backplate and disposed vertically thereon, a hinge attached to the crosssupport of the bracket, a movable crossarm mounted on the hinge, a round and an elongated depression in the movable cross-arm. a fork mounted on the pendulum, the fork including a cross-bar with pointed arms attached thereto, the points of the arms being adapted to be positioned in said depressions in the movable cross-arm.

FRANK D. CAS-L23.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Name Date Arringdale Sept. 20, 1949 OTHER REFERENCES Number 2,482,233 

